Method and apparatus for self-validating checksums in a file system

ABSTRACT

A method for storing a data block, involving storing the data block in a storage pool, obtaining a data block location, calculating a data block checksum for the data block, and storing a first indirect block in the storage pool, wherein the first indirect block comprises the data block location and the data block checksum.

BACKGROUND

A typical operating system includes a file system. The file system provides a mechanism for the storage and retrieval of files and a hierarchical directory structure for the naming of multiple files. More specifically, the file system stores information provided by the user (i.e., data) and information describing the characteristics of the data (i.e., metadata). The file system also provides extensive programming interfaces to enable the creation and deletion of files, reading and writing of files, performing seeks within a file, creating and deleting directories, managing directory contents, etc. In addition, the file system also provides management interfaces to create and delete file systems. File systems are typically controlled and restricted by operating system parameters. For example, most operating systems limit the maximum number of file names that can be handled within their file system. Some operating systems also limit the size of files that can be managed under a file system.

An application, which may reside on the local system (i.e., computer) or may be located on a remote system, uses files as an abstraction to address data. Conventionally, this data is stored on a storage device, such as a disk.

To access a file, the operating system (via the file system) typically provides file manipulation interfaces to open, close, read, and write the data within each file. More specifically, the file system stores data on the storage device by managing the allocation of space within the storage device. Typically, the volume manager provides space which is managed by the file system. Two common types of file system space allocation strategies are known as block-based allocation and extent-based allocation. Block-based allocation creates incremental disk space for each file each time the file is extended (i.e., modified via a write request to add information), whereas extent-based allocation creates a large series of contiguous blocks (i.e., extents) each time the file exhausts the space available in the file's last extent.

When allocating space, both block-based and extent-based allocation use space provided by the volume manager. The volume manager allows multiple physical disks to be used as a single volume (i.e., a virtual disk) to provide larger consolidated storage sizes and simpler management. The volume manager allows users to organize data along volume boundaries (i.e., each volume has physical disk space allocated to the volume such that the volume is tied only to that dedicated physical disk). The volume manager is typically implemented as a separate layer between the physical disks and the file system, and is presented to the user as a virtual disk device. In other words, volume managers organize the collections of physical devices (e.g., disks) into virtual devices. Additionally, the space allocated within the volume manager is handled by the file system. Consequently, the volume manager is not aware of which blocks within the available storage space are in use and which blocks are free for data to be stored.

Further, file systems may be mounted on the virtual disk devices. Thus, physical disks are partitioned and allocated to multiple virtual disk devices, and each virtual disk device is capable of having a file system that exclusively uses that particular virtual disk device. A request to access a file is typically performed by an application, via the file system, using a file name and logical offset. This file name and logical offset (i.e., the manner in which applications express file operation requests) corresponds to a location within the virtual disk device. Subsequently, the request is translated to physical disk space on the storage device by the volume manager, allowing the user of the application to access the data within a particular file.

Files systems and/or volume managers may include a mechanism for determining whether the data stored within the file system has been corrupted or otherwise altered. One such mechanism is a checksum. The checksum corresponds to a set of bits obtained by applying a particular formula (e.g., Message Digest (MD) 5, Fletcher, Cyclic Redundancy Check (CRC) etc.) to the piece of data. The checksum for the particular piece of data is then stored adjacent to the corresponding data in the file system (e.g., using 520 byte sectors).

SUMMARY

In general, in one aspect, the invention relates to a method for storing a data block, comprising storing the data block in a storage pool, obtaining a data block location, calculating a data block checksum for the data block, and storing a first indirect block in the storage pool, wherein the first indirect block comprises the data block location and the data block checksum.

In general, in one aspect, the invention relates to a method for storing a first data block and a second data block, comprising storing the first data block and the second data block in a storage pool, obtaining a first data block location and a second data block location, calculating a first data block checksum for the first data block, calculating a second data block checksum for the second data block, and storing an array of block pointers in an indirect block, wherein the array block of pointers comprises, a first block pointer comprising the first data block location and the first data block checksum, and a second block pointer comprising the second data block location and the second data block checksum.

In general, in one aspect, the invention relates to a method for retrieving data in a data block, comprising obtaining an indirect block comprising a stored checksum and a data block location, obtaining the data block using the data block location, calculating the checksum for the data block to obtain a calculated checksum, retrieving the data from the data block, if the stored checksum equals the calculated checksum, and performing an appropriate action, if the stored checksum is not equal to the calculated checksum.

In general, in one aspect, the invention relates to a method for storing and retrieving a data block, comprising storing the data block, obtaining a data block location, calculating a data block checksum for the data block, storing a block pointer in an indirect block, wherein the block pointer comprises the data block location and the data block checksum, obtaining the indirect block comprising the block pointer, obtaining the data block using the data block location stored in the block pointer, calculating the checksum for the data block to obtain a calculated checksum, retrieving data from the data block, if the data block checksum stored in the block pointer equals the calculated checksum, and performing an appropriate action, if the data block checksum is not equal to the calculated checksum.

In general, in one aspect, the invention relates to a system for storing a data block, comprising a storage pool comprising the data block and a first indirect block, wherein the first indirect block comprises a data block checksum and a data block location, and a storage pool allocator configured to store the data block and the first indirect block in the storage pool.

In general, in one aspect, the invention relates to a computer system for storing a data block, comprising a processor, a memory, a storage device, and software instructions stored in the memory for enabling the computer system under control of the processor, to store the data block in a storage pool, obtain a data block location, calculate a data block checksum for the data block, and store a first indirect block in the storage pool, wherein the first indirect block comprises the data block location and the data block checksum.

In general, in one aspect, the invention relates to a network system having a plurality of nodes, comprising a storage pool comprising the data block and a first indirect block, wherein the first indirect block comprises a data block checksum and a data block location, and a storage pool allocator configured to store the data block and the first indirect block in the storage pool, wherein the storage pool is located on any one of the plurality of nodes, and wherein the storage pool allocator is located on any one of the plurality of nodes.

Other aspects of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a system architecture in accordance with an embodiment of the invention.

FIG. 2 shows a storage pool allocator in accordance with an embodiment of the invention.

FIG. 3 shows a hierarchical data configuration in accordance with an embodiment of the invention.

FIG. 4 shows a flow chart in accordance with an embodiment of the invention.

FIG. 5 shows a hierarchical data configuration in accordance with an embodiment of the invention.

FIG. 6 shows a flow chart in accordance with an embodiment of the invention.

FIG. 7 shows a computer system in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

Specific embodiments of the invention will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency.

In the following detailed description of one or more embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid obscuring the invention.

In general, embodiments of the invention relate to a method and apparatus for using checksums in a file system. More specifically, embodiments of the invention provide a method and apparatus for implementing self-validating checksums in a file system. Further, embodiments of the invention provide a method and apparatus for storing the data and the corresponding checksum in different locations, thereby providing robust fault isolation.

FIG. 1 shows a system architecture in accordance with one embodiment of the invention. The system architecture includes an operating system (103) interacting with a file system (100), which in turn interfaces with a storage pool (108). In one embodiment of the invention, the file system (100) includes a system call interface (102), a data management unit (DMU) (104), and a storage pool allocator (SPA) (106).

The operating system (103) typically interfaces with the file system (100) via a system call interface (102). The operating system (103) provides operations (101) for users to access files within the file system (100). These operations (101) may include read, write, open, close, etc. In one embodiment of the invention, the file system (100) is an object-based file system (i.e., both data and metadata are stored as objects). More specifically, the file system (100) includes functionality to store both data and corresponding metadata in the storage pool (108). Thus, the aforementioned operations (101) provided by the operating system (103) correspond to operations on objects.

More specifically, in one embodiment of the invention, a request to perform a particular operation (101) (i.e., a transaction) is forwarded from the operating system (103), via the system call interface (102), to the DMU (104). In one embodiment of the invention, the DMU (104) translates the request to perform an operation on an object directly to a request to perform a read or write operation at a physical location within the storage pool (108). More specifically, the DMU (104) represents the objects as data blocks and indirect blocks as described in FIG. 3 below. Additionally, in one embodiment of the invention, the DMU (104) includes functionality to group related work (i.e., modifications to data blocks and indirect blocks) into I/O requests allowing related blocks to be forwarded to the SPA (106) together. The SPA (106) receives transactions from the DMU (106) and subsequently writes the blocks into the storage pool (108). The operation of the SPA (106) is described in FIG. 2 below.

In one embodiment of the invention, the storage pool (108) includes one or more physical disks (disks (110A-110N)). Further, in one embodiment of the invention, the storage capacity of the storage pool (108) may increase and decrease dynamically as physical disks are added and removed from the storage pool. In one embodiment of the invention, the storage space available in the storage pool (108) is managed by the SPA (106).

FIG. 2 shows the SPA (106) in accordance with one embodiment of the invention. The SPA (106) may include an I/O management module (200), a compression module (201), an encryption module (202), a checksum module (203), and a metaslab allocator (204). Each of these aforementioned modules in detail below.

As noted above, the SPA (106) receives transactions from the DMU (104). More specifically, the I/O management module (200), within the SPA (106), receives transactions from the DMU (104) and groups the transactions into transaction groups in accordance with one embodiment of the invention. The compression module (201) provides functionality to compress larger logical blocks (i.e., data blocks and indirect blocks) into smaller segments, where a segment is a region of physical disk space. For example, a logical block size of 8K bytes may be compressed to a size of 2K bytes for efficient storage. Further, in one embodiment of the invention, the encryption module (202) provides various data encryption algorithms. The data encryption algorithms may be used, for example, to prevent unauthorized access. In one embodiment of the invention, the checksum module (203) includes functionality to calculate a checksum for data (i.e., data stored in a data block) and metadata (i.e., data stored in an indirect block) within the storage pool. The checksum may be used, for example, to ensure data has not been corrupted.

As discussed above, the SPA (106) provides an interface to the storage pool and manages allocation of storage space within the storage pool (108). More specifically, in one embodiment of the invention, the SPA (106) uses the metaslab allocator (204) to manage the allocation of storage space in the storage pool (108).

In one embodiment of the invention, the storage space in the storage pool is divided into contiguous regions of data, i.e., metaslabs. The metaslabs may in turn be divided into segments (i.e., portions of the metaslab). The segments may all be the same size, or alternatively, may be a range of sizes. The metaslab allocator (204) includes functionality to allocate large or small segments to store data blocks and indirect blocks. In one embodiment of the invention, allocation of the segments within the metaslabs is based on the size of the blocks within the I/O requests. That is, small segments are allocated for small blocks, while large segments are allocated for large blocks. The allocation of segments based on the size of the blocks may allow for more efficient storage of data and metadata in the storage pool by reducing the amount of unused space within a given metaslab. Further, using large segments for large blocks may allow for more efficient access to data (and metadata) by reducing the number of DMU (104) translations and/or reducing the number of I/O operations. In one embodiment of the invention, the metaslab allocator may include a policy that specifies a method to allocate segments.

As noted above, the storage pool (108) is divided into metaslabs, which are further divided into segments. Each of the segments within the metaslab may then be used to store a data block (i.e., data) or an indirect block (i.e., metadata). FIG. 3 shows the hierarchical data configuration (hereinafter referred to as a “tree”) for storing data blocks and indirect blocks within the storage pool in accordance with one embodiment of the invention. In one embodiment of the invention, the tree includes a root block (300), one or more levels of indirect blocks (302, 304, 306), and one or more data blocks (308, 310, 312, 314). In one embodiment of the invention, the location of the root block (300) is in a particular location within the storage pool. The root block (300) typically points to subsequent indirect blocks (302, 304, and 306). In one embodiment of the invention, indirect blocks (302, 304, and 306) may be arrays of block pointers (e.g., 302A, 302B, etc.) that, directly or indirectly, reference to data blocks (308, 310, 312, and 314). The data blocks (308, 310, 312, and 314) contain actual data of files stored in the storage pool. One skilled in the art will appreciate that several layers of indirect blocks may exist between the root block (300) and the data blocks (308, 310, 312, 314).

In contrast to the root block (300), indirect blocks and data blocks may be located anywhere in the storage pool (108 in FIG. 1). In one embodiment of the invention, the root block (300) and each block pointer (e.g., 302A, 302B, etc.) includes data as shown in the expanded block pointer (302B). One skilled in the art will appreciate that data blocks do not include this information; rather data blocks contain actual data of files within the file system.

In one embodiment of the invention, each block pointer includes a metaslab ID (318), an offset (320) within the metaslab, a birth value (322) of the block referenced by the block pointer, and a checksum (324) of the data stored in the block (data block or indirect block) referenced by the block pointer. In one embodiment of the invention, the metaslab ID (318) and offset (320) are used to determine the location of the block (data block or indirect block) in the storage pool. The metaslab ID (318) identifies a particular metaslab. More specifically, the metaslab ID (318) may identify the particular disk (within the storage pool) upon which the metaslab resides and where in the disk the metaslab begins. The offset (320) may then be used to reference a particular segment in the metaslab. In one embodiment of the invention, the data within the segment referenced by the particular metaslab ID (318) and offset (320) may correspond to either a data block or an indirect block. If the data corresponds to an indirect block, then the metaslab ID and offset within a block pointer in the indirect block are extracted and used to locate a subsequent data block or indirect block. The tree may be traversed in this manner to eventually retrieve a requested data block.

In one embodiment of the invention, copy-on-write transactions are performed for every data write request to a file. Specifically, all write requests cause new segments to be allocated for the modified data. Therefore, the retrieved data blocks and indirect blocks are never overwritten (until a modified version of the data block and indirect block is committed). More specifically, the DMU writes out all the modified data blocks in the tree to unused segments within the storage pool. Subsequently, the DMU writes out the corresponding block pointers (within indirect blocks) to unused segments in the storage pool. In one embodiment of the invention, fields (i.e., metaslab ID, offset, birth, checksum) for the corresponding block pointers are populated by the DMU prior to sending an I/O request to the SPA. The indirect blocks containing the block pointers are typically written one level at a time. To complete the copy-on-write transaction, the SPA issues a single write that atomically changes the root block to reference the indirect blocks referencing the modified data block.

Using the infrastructure shown in FIGS. 1-3, the following discussion describes a method for implementing a self-validating checksum in accordance with one embodiment of the invention. FIG. 4 shows a flow chart in accordance with one embodiment of the invention. Initially, the DMU receives a transaction from an application, the operating system (or a subsystem therein), etc. (ST 100). The DMU subsequently groups the transaction into one or more I/O requests (ST 102). The I/O requests are subsequently forwarded to the SPA (ST 104).

In one embodiment of the invention, the transaction includes one or more data blocks, and/or one or more indirect blocks. As noted above, the file system is stored on disk using a hierarchical structure including data blocks and indirect blocks. Thus, for a given set of transactions, the first I/O request includes the data blocks to be written to disk, while subsequent I/O requests include the corresponding indirect blocks containing one or more block pointers. Accordingly, I/O request referenced in ST 104 includes data blocks.

Continuing with the discussion of FIG. 4, the SPA, upon receiving the I/O request including data blocks from the DMU, writes the data blocks into the storage pool (ST 106). The SPA subsequently calculates a checksum for each data block written into the storage pool (ST 108). In one embodiment, the checksum module (203 in FIG. 2) within the SPA is used to calculate the checksum for each data block written into the storage pool. The checksums are subsequently forwarded to the DMU (ST 110). The DMU then assembles the indirect blocks using the checksums (ST 112). Specifically, the DMU places the checksum for a given data block in the appropriate block pointer within the indirect block (i.e., the parent indirect block of the data block). Next, the indirect blocks are forwarded to the SPA (ST 114). Those skilled in the art will appreciate that the aforementioned indirect blocks correspond to the indirect blocks that directly point (via the block pointers) to the data blocks (as opposed to indirect blocks that point to other indirect blocks).

Next, the SPA receives and subsequently writes the indirect blocks into the storage pool (ST 116). A determination is then made whether additional indirect blocks exist to write into the storage pool (i.e., whether the last indirect block written to the storage pool corresponds to the root block) (ST 118). If no additional indirect blocks exist, then the method is complete. However, if additional indirect blocks exist, then the SPA calculates the checksum from each of the indirect blocks written into the storage pool (ST 120). The checksums for each of the indirect blocks is subsequently forwarded to the DMU (ST 122). Steps ST 112 through ST 122 are subsequently repeated until the root block is written into the storage pool.

FIG. 5 shows a hierarchical data configuration in accordance with an embodiment of the invention. Specifically, FIG. 5 shows a detailed view of the data blocks and indirect blocks resulting from using the method described in FIG. 4, in accordance with one embodiment of the invention. Specifically, the file system includes four data blocks (i.e., 408, 410, 412, and 414). Each data block (408, 410, 412, and 414) within the file system has a corresponding checksum (CS408, CS410, CS412, and CS414, respectively) stored in the corresponding block pointer (406A, 406B, 404A, 404B, respectively). Each of the block pointers (406A, 406B, 404A, and 404B) is stored in an indirect block (i.e., 404, 406). Each indirect block (404, 406) also has a corresponding checksum (CS404, CS406, respectively) stored in a corresponding block pointer in a parent indirect block (402). Specifically, block pointer (402A) includes the checksum (CS406) for indirect block (406), and block pointer (402B) includes the checksum (CS404) for indirect block (404). In this particular example, the indirect block (402) is referenced by a root block (400). The root block (400) includes the checksum (CS402) for the array of indirect blocks (402).

FIG. 6 shows a flow chart in accordance with one embodiment of the invention. More specifically, FIG. 6 details a method for reading data in accordance with one embodiment of the invention. Initially, a transaction to read data is received (ST 140). A checksum and a location to the next block (i.e., a data block or an indirect block) stored in the root block are subsequently retrieved (ST 142). The location (e.g., the metaslab ID and offset) is subsequently used to obtain the block (i.e., the data block or the indirect block) (ST 144). The checksum of the retrieved block is then calculated (ST 146). A determination is subsequently made whether the stored checksum is equal to the calculated checksum (ST 148). If the stored checksum is not equal to the calculated checksum, then an appropriate action is performed (e.g., an error message is generated indicating that the data is corrupted) (ST 154).

If the stored checksum is equal to the calculated checksum, then a determination is made whether the retrieved block is a data block (ST 150). If the retrieved block is a data block, then the data is extracted from the data block and presented to the process requesting the data (ST 152). Alternatively, if the retrieved block is not the data block, then the location of the next block (stored within a block pointer within the retrieved block) is obtained (ST 156). The block (data block or indirect block) at the location is subsequently obtained (ST 158). Steps ST 146 through ST 154 are subsequently repeated until either data corruption is encountered (i.e., ST 148) or the data block is encountered (i.e., ST 152).

The invention may be implemented on virtually any type of computer regardless of the platform being used. For example, as shown in FIG. 7, a networked computer system (500) includes a processor (502), associated memory (504), a storage device (506), and numerous other elements and functionalities typical of today's computers (not shown). The networked computer (500) may also include input means, such as a keyboard (508) and a mouse (510), and output means, such as a monitor (512). The networked computer system (500) is connected to a local area network (LAN) or a wide area network (e.g., the Internet) (not shown) via a network interface connection (not shown). Those skilled in the art will appreciate that these input and output means may take other forms. Further, those skilled in the art will appreciate that one or more elements of the aforementioned computer (500) may be located at a remote location and connected to the other elements over a network. Further, the invention may be implemented on a distributed system having a plurality of nodes, where each portion of the invention (e.g., the storage pool, the SPA, the DMU, etc.) may be located on a different node within the distributed system. In one embodiment of the invention, the node corresponds to a computer system. Alternatively, the node may correspond to a processor with associated physical memory.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

1. A method for storing data blocks, comprising: storing a first data block and a second data block in a storage pool, wherein data stored in the storage pool is organized as a hierarchical tree, and wherein the first and second data block are stored on a first level of the hierarchical tree; obtaining a first data block location and a second data block location; calculating a first data block checksum for the first data block; calculating a second data block checksum for the second data block; storing a first indirect block at a first indirect block location on a second level of the hierarchical tree in the storage pool, wherein the first indirect block comprises a first block pointer comprising the first data block location and the first data block checksum and a second block pointer comprises the second data block location and the second data block checksum; calculating a first indirect block checksum for the first indirect block by applying a checksum function to the first indirect block; and storing a second indirect block at a second indirect block location on a third level of the hierarchical tree in the storage pool, wherein the second indirect block comprises the first indirect block location and the first indirect block checksum, wherein each of the first data block location, the second data block location, the first indirect block location, and the second indirect block location are separate physical locations in the storage pool.
 2. The method of claim 1, further comprising: storing a birth value in a birth field in the first block pointer.
 3. The method of claim 1, wherein the storage pool comprises at least one storage device.
 4. The method of claim 1, wherein the storage pool is divided into a plurality of metaslabs.
 5. The method of claim 4, wherein each of the plurality of metaslabs is associated with a metaslab ID.
 6. The method of claim 5, wherein the first data block location comprises the metaslab ID and an offset.
 7. The method of claim 1, wherein storing the first data block and the second data block comprises using a storage pool allocator.
 8. A system for storing data blocks, comprising: a storage pool configured to store data, wherein data stored in the storage pool is organized as a hierarchical tree, comprising: a first data block, a second data block; a first indirect block referencing the first data block and the second data block, wherein the first indirect block comprises a first data block checksum and a first data block location stored in a first block pointer, and a second data block checksum and a second data block location stored in a second block pointer; a second indirect block, comprising a first indirect data block checksum and a first indirect block location; and a storage pool allocator configured to store the first data block and the second data block on a first level of the hierarchical tree, the first indirect block on a second level of the hierarchical tree, and the second indirect block on a third level of the hierarchical tree in the storage pool, wherein each of the first data block location, the second data block location, the first indirect block location, and the second indirect block location are separate physical locations in the storage pool.
 9. The system of claim of claim 8, further comprising: a data management unit configured to assemble the first indirect block and request the storage pool allocator to store the first indirect block.
 10. The system of claim 9, wherein the storage pool comprises at least one storage device.
 11. The system of claim 9, wherein the storage pool is divided into a plurality of metaslabs.
 12. The system of claim 11, wherein each of the plurality of metaslabs is associated with a metaslab ID.
 13. The system of claim 12, wherein the first data block location comprises the metaslab ID and an offset.
 14. A computer system for storing data blocks, comprising: a processor; a memory; a storage device; and software instructions stored in the memory for enabling the computer system under control of the processor, to: store a first data block and a second data block in a storage pool, wherein data stored in the storage pool is organized as a hierarchical tree, and wherein the first and second data block are stored on a first level of the hierarchical tree; obtain a first data block location and a second data block location; calculate a first data block checksum for the first data block; calculate a second data block checksum for the second data block; store a first indirect block at a first indirect block location on a second level of the hierarchical tree in the storage pool, wherein the first indirect block comprises a first block pointer comprising the first data block location and the first data block checksum and a second block pointer comprises the second data block location and the second data block checksum; calculate a first indirect block checksum for the first indirect block by applying a checksum function to the first indirect block; and store a second indirect block at a second indirect block location on a third level of the hierarchical tree in the storage pool, wherein the second indirect block comprises the first indirect block location and the first indirect block checksum, wherein each of the first data block location, the second data block location, the first indirect block location, and the second indirect block location are separate physical locations in the storage pool.
 15. A network system having a plurality of nodes, comprising: a storage pool, configured to store data, wherein data stored in the storage pool is organized in a hierarchical tree, comprising: a first data block, a second data block, a first indirect block referencing the first data block and the second data block, wherein the first indirect block comprises a first data block checksum and a first data block location stored in a first block pointer, and a second data block checksum and a second data block location stored in a second block pointer; a second indirect block, comprising a first indirect data block checksum and a first indirect block location; and a storage pool allocator configured to store the first data block and the second data block on a first level of the hierarchical tree, the first indirect block on a second level of the hierarchical tree, and the second indirect block on a third level of the hierarchical tree in the storage pool, wherein each of the first data block location, the second data block location, the first indirect block location, and the second indirect block location are separate physical locations in the storage pool, wherein the storage pool is located on any one of the plurality of nodes, and wherein the storage pool allocator is located on any one of the plurality of nodes.
 16. A method for retrieving data in a first data block, comprising: obtaining a first indirect block comprising a second indirect block location and a second indirect block checksum, wherein the first indirect block is stored at a first indirect block location on a third level of a storage pool, wherein data stored in the storage pool is organized as a hierarchical tree; obtaining the second indirect block from a second level of the hierarchical tree using the second indirect block location, wherein the second indirect block comprises a first data block pointer and a second data block pointer, wherein the first data block pointer comprises a first data block location and a first data block checksum, and the second data block pointer comprises a second data block location and a second data block checksum, wherein the first and second data block locations are on a first level of the hierarchical tree; applying a checksum function to the second indirect block to obtain a first calculated checksum, comparing the first calculated checksum with the second indirect block checksum; obtaining the first data block using the first data block location when the first calculated checksum equals the second indirect block checksum; applying the checksum function to the first data block to obtain a second calculated checksum; comparing the second calculated checksum with the first data block checksum; and retrieving the data from the first data block when the second calculated checksum equals the first data block checksum, wherein each of the first data block location, the second data block location, the first indirect block location, and the second indirect block location are separate physical locations in the storage pool. 